Many propelled vehicles and work machines, particularly earth working machines, use a continuously variable transmission (CVT) to drive wheels or tracks which propel the vehicle or work machine. A CVT provides an infinite number of transmission ratios to generate an output at any speed in its operating range. One example of a CVT is a hydrostatic transmission consisting of a variable speed hydraulic pump and a hydraulic motor. An example of such a hydrostatic transmission is disclosed in U.S. Pat. Nos. 6,385,970 and 6,424,902 to Kuras et al. The speed output of such a transmission can be continuously varied by controlling the displacement of the hydraulic pump.
Another example of a CVT is an electric motor and inverter as is used in hybrid-electric systems such as in hybrid-electric cars. A hybrid-electric system generally includes an internal combustion engine that is mechanically coupled to drive an electric generator that creates electrical power. The power from the electric generator is then consumed by an electric motor. An inverter contains the power electronics that control the output speed and torque of the electric motor—thus the transmission ratio is adjusted electronically by the inverter. The electric motor provides torque to drive a load, such as the wheels or tracks of a propelled vehicle. Hybrid-electric systems can be used in a variety of applications, such as automobiles, earth-working machines, or other machinery.
One problem encountered in CVT systems is that the inertia of the motor (hydraulic or electric) and the associated system components can be quite large. When the inertia is large, the acceleration response of the motor is limited. In other words, if the inertia is large, it will be difficult to accelerate the motor quickly unless the engine is sufficiently powerful. A limited motor acceleration response may be unacceptable, especially for certain applications such as construction machines like wheel loaders or utility tractors driven by the CVT system. More power can be provided to the motor simply by installing a more powerful engine and generator. This is often undesirable, however, because a more powerful engine may also be larger, heavier, and result in a less efficient system.
U.S. Pat. No. 6,726,594 to Mizuno discloses a control system for a vehicle. A controller detects whether an acceleration of a vehicle is requested. If acceleration is requested, a final target engine output power is calculated based on the accelerator pedal position and the vehicle speed. The engine speed at which the target output power can be generated with the minimum fuel consumption is calculated. A target engine torque is calculated based on the target engine output power and engine speed. A transient operating point is then determined by calculating ranges of engine torque and engine speed which can be achieved within a predetermined time. The controller then controls the speed ratio of a continuously variable transmission so as to operate the engine at the transient operating point.
Controlling the speed ratio of a CVT may not provide sufficient acceleration if the rate of fuel consumption in the engine is not increased to boost engine power output. What is needed is a system and method for improving motor acceleration in a CVT system without requiring the installation of a larger engine. What is further needed is a system and method for boosting power to a motor in a CVT system without damaging the engine. The disclosed system is directed to satisfying one or more of the existing needs in the industry.